1. Field of the Invention
The invention is based on a process for producing a current limiter having a high-temperature superconductor, and on a current limiter,
2. Discussion of Background
EP 0,592,797 A2 discloses a process for producing a circular-cylindrical high-temperature superconductor with high current-carrying capacity for screening purposes and for inductive current limiters, in which, for use as magnetic screening, the silver which is put in the casting mold acts as a highly conductive bypass for the high-temperature superconductor. In this case Bi2Sr2CaCu2O8 powder, for example, either in dry form or as a suspension consisting of powder, organic solvent and binder, is introduced at room temperature via a feed trough into the casting mold. After the binder has been baked out at 500xc2x0 C.xc2x150 K., partial melting of the high-temperature superconductor takes place, with subsequent cooling and slow annealing up to three times in an oxygen atmosphere. In this case, self-supporting superconducting cylinders having a wall thickness of 2.5 mm are peeled from the 200 xcexcm thick silver mold, while superconducting cylinders having a wall thickness of 50 xcexcm cannot be separated from a 50 xcexcm thick silver layer, which is located on a 1 mm thick Ni-Cr or Fe-Ni alloy, as a support. This silver-coated Ni-Cr or Fe-Ni layer then acts as mechanical and electrical reinforcement in current limiters.
A disadvantage in this case is that a 200 xcexcm thick silver mold for high-temperature superconductors is relatively expensive. Silver-coating Ni-Cr or Fe-Ni layers are demanding to produce. Furthermore, the hot high-temperature superconductor makes fine holes in thin silver layers ( less than 100 xcexcm). Liquid high-temperature superconductor emerges through these holes and reacts with the support of the silver layer, so that the high-temperature superconductor becomes contaminated.
DE 4,434,819 C1 discloses a current limiter which, on each of the two sides of a 1 mm thick sheet of insulator made of firing resin reinforced with glass fibers or of an MgO ceramic plate, has a composite conductor consisting of a high-temperature superconductor and a normal conductor made of a metal alloy or metal that is ductile at room temperature. A chemically insulating buffer layer of 1 xcexcm-5 xcexcm thick silver is arranged between these composite conductors and the insulator. Each composite conductor is designed with a meander shape, and they are connected to one another in such a way that a current flows in opposite direction in composite-conductor subunit bands of the composite conductors lying directly opposite one another, so that the components of the self-magnetic field perpendicular to the plane of the bands compensate one another and the current limiter modules have low inductance and low loss. The surface-related contact resistively between a main surface of the high-temperature superconductor and of the normal conductor is  less than 1 mxcexa9cm2, preferably xe2x89xa610 xcexcmxcexa9cm2. In the case when the superconductor sheets have been fired in a silver mold which is made of a 100 xcexcm thick silver sheet that is removed after the firing, the sheet insulator is applied to the silver buffer layer using an adhesive.
For current limitation applications, the silver substrate needs to be removed because of its insufficient electrical resistance. In order to provide electrical stabilization, the high-temperature super-conductor is subsequently re-coated with a high-resistance normal electrical conductor. This production process is relatively elaborate and quite expensive because of the amounts of silver that are required.
Firing substrates on a nickel-based alloy having a thin protective coating of silver or of MgO ceramic do not need to be removed after the high-temperature superconductor has been produced, and serve as mechanical stabilization for this superconductor. The thickness of the normal conductor is in the range of 10 xcexcm-15 xcexcm, and that of the high-temperature superconductor is in the range of 0.3 mm-3 mm.
For current limiters, it is desirable to produce superconducting hollow cylinders or sheets that are as thin-walled as possible in order to keep their AC losses as low as possible. At the same time, the superconductor needs electrical stabilization whose resistance is neither too low nor too high.
The only substrates which do not lead to reduction of the superconducting properties of the high-temperature superconductor are silver, gold and MgO. Although an Ag/Au alloy containing 5% Au would have a higher resistance than pure silver, it is much more expensive than silver. MgO substrates cannot be used directly as electrical stabilization, and MgO parts of the required quality are also more expensive than pure silver. Reducing the layer thickness of the silver foil used as substrate would both reduce the cost of the substrate and increase its electrical resistance, so that it could be used directly as electrical stabilization.
Accordingly, one object of the invention, is to provide a novel process for producing a current limiter having a high-temperature superconductor, and a novel current limiter of the type mentioned at the start, such that it is simpler and less expensive to produce.
One advantage of the invention consists in that the silver foil used. for firing the high-temperature superconductor only needs to be about 20 xcexcm thick. The high-temperature superconductor does not become contaminated, nor does it bond with the support of the silver foil. The silver foil may subsequently be left as electrical stabilization on the superconductor.
According to one advantageous refinement of the invention, relatively flexible sheets can be bent to form tubes and can be electrically connected to one another at the seams.